Recently, a digital information recording and reproducing device, which is a so-called digital recorder, has been developed. This device is able to convert a sound signal obtained by a microphone or the like to a digital signal and record it in a memory such as an IC memory. When reproducing the sound signal, the device reads the digital signal from the memory and converts it to an analog signal to output it from an output means such as a speaker.
In such a digital information recording and reproducing device, a coding technique to compress the volume of data efficiently for saving the space of the memory is applied to reduce a volume of data as little as possible. A method of code excited linear predictive coding with an adaptive code book is widely adopted as a highly efficient data compressing technique. This method is recognized as being able to obtain reproduced sounds of comparatively high quality at a bit rate of 8 kbps. Therefore, many applications of the method have been developed, especially, in the field of digital mobile communication.
In the digital information recording and reproducing device, a reproduced position is defined by designating an address for the memory. For instance, a change of the reproduced position by skipping forward (selecting forward address: FF Mode), skipping backward (selecting a backward address: REW Mode) or repeating (selecting the same address repeatedly after a period of time: Repeat Mode) is completed by the counting operation of an address counter.
However, the adaptive code book is created based on the past excitation signal. Therefore, when the reproduced position is changed, the contents of the adaptive code book would not be related to those of the adaptive code book created based on the new excitation signal at the changed position so that a strange sound is produced. Hence, there is the problem that quality of the reproduced sound is poor.
If the contents of the adaptive code book are cleared in accordance with the change of the reproduced position, the strange sounds will be less produced. However, when the reproduced position is on a constant part of voiced sound, a pulse signal of the voiced sound cannot be formed. Hence, the quality of the reproduced sound becomes poor.
Further, the above described sound recording and reproducing device is also used in the fields of the study of foreign languages or dictation. In such applications, it is desirable to be able to vary the speed of reproduction. As an example, Japanese Laid-Open Patent Application Publication No. Hei 2-93700 discloses a device.
Japanese Laid-Open Patent Application Publication No. Hei 2-93700 discloses a sound decoding device using multi-pulse speech coding and decoding with pitch prediction. When making the speed slow, the device extends a frame length corresponding to a ratio of the change of reproduction speed and supplements the excitation signal in the extended part of the frame with null elements. When making the speed fast, the device reduces the frame length corresponding to a ratio of the change of reproduction speed and discards the remainder part. Thus, the reproduction speed can be changed only by making little additional changes to the sound decoding device.
However, the sound decoding device disclosed in Japanese Laid-Open Patent Application Publication No. Hei 2-93700 only changes the frame length at the time of decoding, so that the reproduced sound becomes very hard to listen to because of an improper interval of the reproduced sound.